services: Handle undefined error event

When trying to connect to an address where no server is running it seams that the error event can be undefined. This will result in an internal error since `event.message` cannot be accessed.

Noticed while upgrading to the latest version of `@jupyterlab/services`: https://github.com/nteract/hydrogen/pull/908

packages/services/src/serverconnection.ts

@@ -328,8 +328,8 @@ namespace Private {
       delegate.reject({ xhr, event, request, settings, message: 'Aborted' });
     };
 
-    xhr.onerror = (event: ErrorEvent) => {
-      delegate.reject({ xhr, event, request, settings, message: event.message });
+    xhr.onerror = (event?: ErrorEvent) => {
+      delegate.reject({ xhr, event, request, settings, message: event ? event.message : 'Errored' });
     };
 
     xhr.ontimeout = (event: ProgressEvent) => {